1. Field of the Invention
The present invention relates to a display, a method for driving a display, and electronic apparatus, and particularly to a flat-type (flat-panel) display in which pixels, each including an electro-optical element, are arranged on rows and columns (in a matrix), a method for driving the display, and electronic apparatus including the display.
2. Description of the Related Art
In recent years, in the field of displays for image displaying, flat-type display obtained by arranging pixels (pixel circuits) each including a light-emitting element in a matrix is prevailing rapidly. As the flat-type display, an organic electroluminescence (EL) display is being developed and commercialized. The organic EL display includes organic EL elements as the light-emitting elements of the respective pixels. The organic EL element is a so-called current-driven electro-optical element whose light-emission luminance varies depending on the value of the current flowing through the element, and is based on a phenomenon that light emission occurs in response to electric field application to an organic thin film.
The organic EL display has the following features. Specifically, the organic EL display has low power consumption because the organic EL element can be driven by an application voltage lower than 10 V. Furthermore, because the organic EL element is a self-luminous element, the organic EL display provides higher image visibility compared with a liquid crystal display, which displays an image by controlling, for each pixel including a liquid crystal cell, the intensity of light from a light source (backlight) by the liquid crystal cell. In addition, the organic EL display does not need to have an illuminating unit such as a backlight, which is necessary for the liquid crystal display, and therefore reduction in the weight and thickness of the organic EL display can be easily achieved. Moreover, the response speed of the organic EL element is as high as several microseconds, which causes no image lag in displaying of a moving image by the organic EL display.
As the drive system for the organic EL display, a simple-(passive-)matrix system or an active-matrix system can be employed, similar to the liquid crystal display. However, a display of the simple-matrix system involves a problem that it is difficult to realize a large-size and high-definition display since the light-emitting period of the light-emitting element is decreased due to the increase of the scanning line (i.e. number of pixels) although the configuration thereof is simple.
For this reason, in recent years, a display of the active-matrix system is being actively developed in which the current flowing through an electro-optical element is controlled by an active element, such as an insulated gate field effect transistor (typically, thin film transistor (TFT)), provided in the same pixel circuit as that including this electro-optical element. In the display of the active-matrix system, the electro-optical element continues light emission over the one-frame period. This easily realizes a display having a large size and high definition.
It is generally known that the I-V characteristic (current-voltage characteristic) of an organic EL element deteriorates as the time elapses (so-called age deterioration). In a pixel circuit including an N-channel TFT as a transistor for driving an organic EL element by current (hereinafter, referred to as a “drive transistor”), the organic EL element is connected to the source side of the drive transistor. Therefore, the age deterioration of the I-V characteristic of the organic EL element leads to change in the gate-source voltage Vgs of the drive transistor, which results in change in the light-emission luminance of the organic EL element.
A more specific description will be made below about this point. The source potential of the drive transistor is dependent on the operating point of the drive transistor and the organic EL element. The deterioration of the I-V characteristic of the organic EL element varies the operating point of the drive transistor and the organic EL element. Therefore, even when the same voltage is applied to the gate of the drive transistor, the source potential of the drive transistor varies. This varies the source-gate voltage Vgs of the drive transistor, which changes the value of the current flowing through the drive transistor. As a result, the value of the current flowing through the organic EL element also changes, which varies the light-emission luminance of the organic EL element.
Furthermore, a pixel circuit employing a poly-silicon TFT involves, in addition to the age deterioration of the I-V characteristic of the organic EL element, changes over time in the threshold voltage Vth of the drive transistor and the mobility μ in the semiconductor thin film serving as the channel of the drive transistor (hereinafter, referred to as “the mobility of the drive transistor”), and differences in the threshold voltage Vth and the mobility μ from pixel to pixel due to variation in the manufacturing process (variation in transistor characteristics among the respective drive transistors).
If the threshold voltage Vth and mobility μ of the drive transistor are different from pixel to pixel, variation in the value of the current flowing through the drive transistor arises on a pixel-by-pixel basis. Therefore, even when the same voltage is applied to the gate of the drive transistor among the pixels, variation in the light-emission luminance of the organic EL element among the pixels arises, which results in lowered uniformity of the screen.
To address this problem, there has been proposed a configuration aiming to allow the light-emission luminance of the organic EL element to be kept constant without being affected by the age deterioration of the I-V characteristic of the organic EL element and changes over time in the threshold voltage Vth and mobility μ of the drive transistor. Specifically, in this configuration, each of pixel circuits is provided with a compensation function against change in the characteristic of the organic EL element, and correction functions for correction against variation in the threshold voltage Vth of the drive transistor (hereinafter, referred to as “threshold correction”) and correction against variation in the mobility μ of the drive transistor (hereinafter, referred to as “mobility correction”) (refer to e.g. Japanese Patent Laid-Open No. 2006-133542 (hereinafter referred to as Patent Document 1)).
By thus providing each pixel circuit with the compensation function against change in the characteristic of the organic EL element and the correction functions against variation in the threshold voltage Vth and mobility μ of the drive transistor, the light-emission luminance of the organic EL element can be kept constant without being affected by the age deterioration of the I-V characteristic of the organic EL element and changes over time in the threshold voltage Vth and mobility μ of the drive transistor.
The compensation function against change in the characteristic of the organic EL element is implemented through the following series of circuit operation. Initially, at the timing when a video signal supplied via a signal line is written by a write transistor and is held in a holding capacitor connected between the gate and source of the drive transistor, the write transistor is turned to the non-conductive state to thereby electrically separate the gate electrode of the drive transistor from the signal line for turning the gate electrode to the floating state.
If the gate electrode of the drive transistor enters the floating state, in response to change in the source potential Vs of the drive transistor, the gate potential Vg of the drive transistor also changes in linkage with (in such a manner as to follow) the change in the source potential Vs, due to the connection of the holding capacitor between the gate and source of the drive transistor. This is a bootstrap operation. Due to the bootstrap operation, the gate-source voltage Vgs of the drive transistor can be kept constant. Thus, even when the I-V characteristic of the organic EL element changes over time, the light-emission luminance of the organic EL element can be kept constant.
In this bootstrap operation, the ratio of the rise amount ΔVg of the gate potential Vg of the drive transistor to the rise amount ΔVs of the source potential Vs thereof (hereinafter, referred to as a bootstrap ratio Gbst) is an important factor. Specifically, if this bootstrap ratio Gbst is low, the gate-source voltage Vgs of the drive transistor becomes lower than the voltage obtained at the timing when the video signal is held in the holding capacitor.
The low bootstrap ratio Gbst is equivalent to the fact that the rise amount ΔVg of the gate potential Vg is small with respect to the rise amount ΔVs of the source potential Vs. Therefore, the gate-source voltage Vgs becomes lower. This leads to failure in ensuring of the current necessary as the drive current to be applied to the organic EL element, i.e., the current corresponding to the video signal written by the write transistor, which results in luminance lowering. Thus, luminance unevenness occurs, which causes the deterioration of the image quality.
The bootstrap ratio Gbst depends on the capacitance of the holding capacitor and the capacitances of parasitic capacitors attaching to the gate of the drive transistor. The higher the capacitances of these capacitors are, the higher the bootstrap ratio Gbst is (details of this point will be described later). The capacitance of the parasitic capacitances depends on circuit elements, such as a transistor, connected to the gate electrode of the drive transistor. If the number of elements included in the pixel circuit is reduced and consequently the number of transistors connected to the gate electrode of the drive transistor is decreased, the capacitance of the parasitic capacitors becomes lower correspondingly.
Therefore, increasing the capacitance of the holding capacitor is effective to increase the bootstrap ratio Gbst. The capacitance of the holding capacitor is in proportion to the area of two metal electrodes that form the holding capacitor and are disposed to face each other, and is in inverse proportion to the distance between these two metal electrodes. Therefore, the capacitance of the holding capacitor can be increased by increasing the area of two metal electrodes or decreasing the distance between two metal electrodes. Because there is a limit to the decreasing of the distance between two metal electrodes, the increasing of the area of two metal electrodes, i.e., increasing of the size of the holding capacitor, is preferentially attempted.
However, because the holding capacitor is formed under the condition of the limited pixel size, there is also a limit to the increasing of the size of the holding capacitor. On the contrary, the recent trend toward miniaturization of the pixel size accompanying definition enhancement makes it difficult to increase the bootstrap ratio Gbst through the increasing of the size of the holding capacitor.
As another scheme, it would be available to ensure the drive current corresponding to the video signal by, instead of increasing the bootstrap ratio Gbst, originally designing a large current as the drive current to be applied to the organic EL element via the drive transistor in anticipation of the voltage decrease corresponding to the bootstrap ratio Gbst. However, this scheme involves a problem of an increase in the power consumption.